WORKING PRINCIPLE
A thermocouple comprises an electrical circuit formed by two metal conductors of different metals soldered together at the ends. When there is a difference in temperature between the two joints, due to the Seebeck effect, a loop of current is generated and then, if one of the two joints is opened, an electromotive force (emf).The polarization and intensity of the electromotive force depends solely on the type of metals used and the temperature to which the joints are subjected. The joint exposed to the temperature to be measured is called the hot junction or the measuring junction while the joint between the thermocouple conductors and the measuring circuit is called the cold junction or the reference junction.
To measure a temperature with a thermocouple the reference junction must be at a given temperature (normally 0°) so that the emf generated depends solely on the temperature of the measuring junction.
MEASURING METHODS
The methods for carrying out measurements with thermocouples can generally be divided into two types.The first, is generally used in industrial fields where extreme precision is not necessary. In this case the thermocouple is connected directly to the measuring device using compensated or extension cables. In this case the compensation of the reference junction is carried out directly by the measuring device which, measuring the junction temperature with other types of sensors, electronically modifies the thermocouple signal so that it is only dependent on the temperature of the measuring junction and thus the temperature to be measured.
The second type enables highly accurate measurements to be obtained and for this reason is used almost exclusively in laboratory applications.
In this case the temperature of the reference junction is maintained at a given and constant temperature (normally the melting point of ice 0°C) through manual or automatic procedures in order to compensate the electromotive force measured by the measuring device with that corresponding to the measuring junction.
